The parents of critically ill baby Charlie Gard have dropped their legal bid to send him to the United States for an experimental treatment after new medical tests showed that the window of opportunity to help him had closed.

Chris Gard and Connie Yates wept as they withdrew their appeal during a London High Court hearing, signalling the end of a legal saga that had stretched for months.

Charlie Gard timeline

Charlie Gard was born in August 2016

He was taken to hospital in September and diagnosed with mitochondrial DNA depletion syndrome

In March 2017, hospital bosses asked a judge to rule that life support treatment should stop

In July 2017, Chris Gard and Connie Yates returned to the High Court

The couple's attorney, Grant Armstrong, said recent medical tests on 11-month-old Charlie showed the baby had irreversible muscular damage, and the new treatment would not help.

"For Charlie, it is too late. The damage has been done," Mr Armstrong said.

"Charlie has waited patiently for treatment. Due to delay, that window of opportunity has been lost."

Mr Armstrong said the news had left Charlie's parents extremely distressed and they now "wish to spend the maximum amount of time they have left with Charlie".